This invention relates to shaping glass sheets and particularly to an improved apparatus for press bending glass sheets into desired shapes. Glass sheets are fabricated into bent shapes for such uses as curved windows for automobiles, motorboats, and other vehicles, as cover windows for instrument panels and television tubes, and other articles, by heating a succession of glass sheets while the latter are conveyed through a furnace of tunnel-like configuration, and removing the sheets one at a time when each sheet in turn has attained its deformation temperature. Each heat softened sheet is engaged between a pair of opposed shaping molds having complementary curvatures conforming to the shape desired for the bent glass sheets. If desired, the bent sheets are tempered by sudden chilling before the temperature is lowered below an elevated temperature suitable for glass tempering immediately following the press bending operation. The present invention is particularly suitable for press bending sheets that are relatively thin. The term "thin", when referring to the thickness of glass sheets for the purposes of this invention, is limited to a maximum thickness of 4.5 millimeters.
Glass sheets are usually gripped near their upper edges by one or more pairs of self-closing tongs during this thermal treatment incidental to press bending and tempering. The tongs are suspended from carriages which are conveyed along a horizontal path of a conveyor that extends lengthwise through the enclosed furnace and the glass shaping station.
During the heating that softens the glass before it is shaped, the tongs provide gripping forces opposing one another through the thickness of the glass sheet. Each glass sheet is normally freely suspended in a vertical plane from the tongs. Normally, more than one pair of tongs is needed to support a flat glass sheet for thermal treatment. The number of tongs depends upon the length and mass of the glass sheet undergoing treatment.
Many devices have been developed to insure that flat glass sheets are properly suspended, usually from a plurality of tongs. Proper loading reduces the likelihood that each tong will apply a twisting force that distorts the glass and imparts a defect known as a kink therein. When a glass sheet is loaded properly, the gripping force applied thereto by each pair of tongs is substantially equal to that applied by each other pair of tongs.
Each pair of tongs exerts a pull in a substantially vertical direction at each glass gripping region gripped by a tong pair. When the glass sheet becomes heat softened, its viscosity is lowered. Its low viscosity at elevated temperatures permits the glass to distort in the vicinity of each gripping region and form a convex bump in the upper edge of the supported glass sheet. The effect of this pull is minimized when the portion of glass weight borne by each pair of tongs is minimized. An optimum condition is obtained when the number of tongs used increases and an equal portion of the glass weight is borne by each tong pair. However, the complexity of loading the glass sheet increases with the number of tongs used. Therefore, a compromise is usually made to limit the number of tongs used to the minimum needed to support the weight of the glass sheet treated and obtain an upper edge distortion within acceptable tolerance limits.
When a plurality of tongs is employed to provide substantially equal gripping forces for tempering flat glass sheets, the defects due to kinking and pulling are reduced substantially. However, the prior art criteria for supporting flat glass sheets probably are insufficient to avoid kinking and pulling when the flat glass sheets are shaped into curved configurations, particularly when gripped by tongs for shaping the sheets about a bending curve having a horizontally disposed axis of curvature.
When glass sheets are bent in such a manner, the upper and lower glass sheet portions are displaced horizontally from the vertical plane occupied by the flat glass. The tongs supporting the upper glass sheet edge portions are rotated out of plumb and pulled into the notched portion of one or the other press bending molds. This movement tends to kink the softened glass because the notched portions provide clearance for tong movement so that each pair of tongs is free to move into the spaces provided by the notched portions while the press bending molds engage the glass sheet while the glass remains softened by heat. Such tong movement meets only modest resistance to movement from the heat-softened glass, which reacts more readily to move into the shape impressed by the molds. Each pair of tongs that is slower to change its position as the press bending molds close against the opposite glass sheet surfaces causes a kink in the glass, particularly when the glass is thin, that is, having a thickness of 4.5 millimeters or less. When glass sheets are bent about vertical axes of bending, the longitudinal end portions of the bent glass sheets are displaced to one side of the vertical plane in which the flat glass is initially suspended from the freely hanging tongs while the central portions are displaced to the other side of said vertical plane. The prior art has suggested disposing two pairs of tongs along the length of the glass sheet at the two locations where the flat glass position is intersected by the curved glass sheets. Such a technique is only suitable for relatively small and light pieces of glass having a mass sustainable by only two pairs of tongs. However, the size of present day glass sheets used in automobiles in such that more than two pairs of tongs are required to minimize tong distortion. Hence, this proposed tong arrangement is suitable only for handling glass sheets of limited size.
Prior to the present invention, a yieldable device was attached to a press bending mold in alignment with and recessed behind its notched portion so as to provide a yieldable device for contacting the glass engaging elements of freely suspended tongs to limit the rate of tong displacement from its plumb line or freely hanging position. This prior art technique was introduced to limit the force the tongs apply against the heat softened glass portions they engage that tend to dimple the engaged portions. Such a spring loaded tong restrictor of the prior art depends on its spring tension to reduce the severity of any tong kinks imparted to the portion of the glass facing the notched portion of the pressing mold. Such spring loaded members that engage the glass-engaging elements of the tongs have been found to be useful with heavier or thicker glass sheets of commercial thicknesses prevalent prior to the present invention. However, the recent energy shortage has provided an impetus for the automotive industry to reduce the weight of automobiles. One method of weight reduction has been accomplished by reducing the thickness of the glass windows included in automobiles. Thus, the spring tensioned members engaging the glass engaging elements of the tongs, which were adequate for use with thicker glass in the past, did not eliminate the glass kinking in the area facing the notched portion of the press bending molding when the need arose to provide shaped glass sheets of thinner dimensions.